my older sister
Humble term used to refer to one's own older sister when speaking to someone outside the family. Not used for the listener's sister.
愚姉は来月結婚します。
My older sister is getting married next month.
Compound of 愚 (gu, 'foolish, humble prefix') and 姉 (shi, 'older sister'). The prefix 愚 is used to humble one's own family members.